1. Field of the Invention
This invention relates to a laser apparatus for performing a laser oscillation upon supplying an excitation light to active lasing substances placed inside an optical fiber or an optical waveguide.
2. Description of Related Arts
While, in arts of optical telecommunication and optical fabrication technology, development of high-power, inexpensive laser apparatuses has been expected, optical fiber laser apparatuses and optical waveguide type laser apparatuses have previously been known as highly possible apparatuses to satisfy such expectations. Such optical fiber laser apparatuses and optical waveguide type laser apparatuses are capable of relatively easily rendering an oscillation transverse mode into a single mode by controlling a core diameter, a refractive index difference between a core and a clad, and the like. Confinement of light in a high density can more activate mutual actions between active lasing substances and light. When the length of the mutual actions is made longer, laser beam can be produced with a high efficiency and a dimensionally high quality. Thus, a high-quality, relatively inexpensive laser beam can be obtained.
To realize further higher power and efficiency of the laser beam, excitation light is necessarily guided with a high efficiency to a doped region (ordinarily, a core portion) of emission centers such as active lasing ions, dyes, or the like (hereinafter referred to as "active lasing substances") for optical fibers or optical waveguides. However, where a core diameter is designed to meet the waveguide condition for the single mode, the core diameter is generally limited to 15 to 16 micrometers or less of the doped region (ordinarily, a core portion) at which active lasing ions are doped, and therefore, it is generally difficult to guide the excitation light to the portion of that diameter with a high efficiency. To overcome this problem, for example, a double clad type fiber laser has been proposed.
FIG. 6 illustrates a double clad type fiber laser. As shown in FIG. 6, the double clad type fiber laser includes a clad portion 16, and a second clad portion 17 placed outside the clad portion 16 and formed of a transparent material having a lower refractive index than that of the clad portion 16. Excitation light 13 introduced from an end is confined in the clad portion 16 and a core portion 15 by total reflection created by a refractive index difference between the second clad portion 17 and the clad portion 16. The confined excitation light is controlled to repeatedly pass the doped region (ordinarily, the core portion 15) of the active lasing substances, thereby rendering the excitation light gradually absorbed by the active lasing substances. The double clad type fiber laser thus constructed can obtain a high power laser beam.
In the case of such a double clad type fiber laser, however, if the cross section of the inner clad portion is circle, there arises a problem that the mode may cause an absorption saturation in which excitation light selectively passing around the doped region (ordinarily, the core portion) of the active lasing substances is well absorbed by the active lasing substances whereas other regions show very low absorption efficiency. Although a rectangular inner clad portion has been devised, a fiber having a cross-sectional shape other than circle is hardly manufactured generally and may reduce a mechanical strength of the fiber.
Meanwhile, optical waveguide type lasers, due to manufacturing methods for the lasers, cannot make the waveguide length longer as much as that of an optical fiber, and it is difficult for optical waveguide type lasers to have a refractive index profile in a cross-sectional direction as complicated as the double clad type lasers have. To solve such problems with an optical waveguide laser, a method has been proposed in which excitation light is guide from a side face of the doped region (ordinarily, a core portion) of the active lasing substances for the optical waveguide, as disclosed in Japanese Laid-open Patent Publication No. Hei 4-51,027 and Japanese Laid-open Patent Publication No. Hei 3-3,283. When excitation light is guided from the side face to the doped region (ordinarily, a core portion) of the active lasing substances, excitation energy of an extremely large amount, in comparison with a method in which excitation light is slightly introduced in a cross-sectional direction of the waveguide, can be pumped in the waveguide because the waveguide length (L) is very long in comparison with a diameter (d) of the doped region of the active lasing substances, namely, L/d is 10.sup.6 or greater. Similarly, with an optical fiber laser, if excitation light could have been introduced from a side face, areas for introducing excitation light can be increased as much as wanted, and therefore, excitation light energy 10.sup.6 to 9 times larger than that of the method in which excitation light is introduced in the cross-sectional direction, or 10.sup.3 to 10.sup.6 times larger than that of the double clad type excitation method can be introduced in principle.
With the conventional side face excitation method thus described, however, when a single excitation light supplied from the side face is closely observed, this excitation light passes only one time the core serving as a doped region of the lasing active region. This core generally has a narrow width of about 50 to 60 micrometers. Therefore, it is very difficult to render the active lasing substances absorb the whole energy of the excitation light during one time passage over the narrow width. Accordingly, the conventional side face excitation method has a defect that much excitation light becomes useless.
This invention is devised on the basis of the background thus described, and it is an object of the invention to provide a laser apparatus capable of very effectively converting excitation light to laser oscillation light.